Method and apparatus for treating a steel article

ABSTRACT

A method for forming and treating a steel article of a high strength and ductile alloy. The method includes the steps of providing a starting steel composition for the steel article, preheating the composition, heating the starting material to a peak temperature range in less than forty seconds, holding the heated steel composition at the peak temperature range for between two and sixty seconds, quenching the heated steel composition from the peak temperature range to below 177° C. (350° F.) at a temperature rate reduction of 200 to 3000° C./sec (360 and 5400° F./sec), removing residual quench media from the surface of the quenched steel composition, tempering the quenched steel composition at a temperature of 100 to 704° C. (212 to 1300° F.); and air cooling the tempered steel composition to less than 100° C. (212° F.) to form a steel having desired mechanical properties.

RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S.application Ser. No. 14/460,825 filed on Aug. 15, 2014, which is acontinuation-in-part of and claims priority to U.S. Pat. No. 8,894,781issued on Nov. 25, 2014, which is a continuation of and claims priorityto U.S. patent application Ser. No. 13/838,693, filed on Mar. 15, 2013,which claims priority to U.S. Provisional Patent Application No.61/661,540, filed on Jun. 19, 2012. The contents of the foregoingapplications are incorporated by reference in their entirety.

BACKGROUND AND SUMMARY

This invention relates to the heat treatment of steel articles, and inparticular, the invention relates to induction heating, quenching, andtempering of steel sheets.

In order to improve the mechanical properties of metal articles, metalis typically subjected to time consuming, and therefore costly, heattreatment processes. To increase the hardness of steel, a steel articlemay be subjected to a heating cycle at or above a temperature of themetal's critical temperature, followed by quenching the metal article.This process typically results in the formation of a martensiticmicrostructure in steels. Martensitic microstructures, while relativelyhard, are also known to be relatively brittle, and with less ductility.To increase the ductility of martensitic microstructures, such steelsare often tempered, or heated to a temperature below the steel'scritical temperature, whereby stresses built up in the steel duringquenching are reduced. Such heating, quenching, and tempering processesare typically long to conduct, and accordingly, expensive.

Generally, in processing steel, and, more specifically, in forminganti-ballistic armor, it has until now been difficult to achieve a metalproduct having a combination of strength and ductility which could bemanufactured without high cost, including extensive heat treatment time.For example, a metal article should be able to resist penetration byarmor piercing ammunition as well as fragments from improvised explosivedevices, including explosively formed projectiles. We have found amethod and apparatus for heat treating, quenching, and tempering a steelarticle whereby the article has desirable mechanical and microstructureproperties, including properties which may be useful in acting asanti-ballistic armor or in other applications which may require a steelsheet having high hardness in combination with high ductility.

Disclosed is a method for treating a steel article to form a high yieldstrength and ductile alloy comprising the steps of:

(a) providing a steel composition having a material thickness no greaterthan 0.5 inches (12.7 mm), having an initial microstructure of at leastferrite and pearlite, and having a composition of, by weight,

carbon between 0.25 and 0.55%,

silicon between 0.15 and 0.35%,

manganese between 0.40 and 1.0%,

chromium between 0.40 and 1.10%,

nickel less than 4.5%,

molybdenum between 0.15 and 0.35%,

sulfur less than 0.040%,

phosphorus less than 0.035%, and

balance iron and other elements and compounds in making steel;

(b) preheating the provided steel composition to not more than 594° C.(1100° F.);

(c) heating the provided steel composition to a peak temperature ofbetween 800° C. (1472° F.) and 1150° C. (2102° F.) in less than fortyseconds;

(d) holding the heated steel composition at the peak temperature rangefor between two and twenty seconds;

(e) quenching the heated steel composition from the peak temperaturerange to below 117° C. (350° F.) at a temperature rate reduction ofbetween 200 and 3000° C./sec (360-5400° F./sec);

(f) tempering the quenched steel composition at a temperature from 100°C. to 704° C. (212-1300° F.) for less than ninety minutes;

(g) air cooling the tempered steel composition to less than 100° C.(212° F.) to form a steel article having at least 80% martensite and upto 5% bainite by weight, a yield strength of at least 160 Ksi (1100MPa), and a total elongation between 5% and 22%.

Additionally, the air cooled steel composition may have a V₅₀ protectionballistic limit at 30° obliquity angle at least 2300 feet per second(701 m/s) with a 0.30 caliber armor piercing round for a thickness of0.25 inches (6.35 mm).

The air cooled steel composition may have a microstructure with no morethan 1% bainite, by weight.

Alternatively, disclosed is a method for treating a steel article toform a high yield strength and ductile alloy comprising the steps of:

(a) providing a steel composition having a material thickness no greaterthan 0.5 inches (12.7 mm), having an initial microstructure of at leastferrite and pearlite, and having a composition of, by weight,

carbon between 0.25 and 0.55%,

silicon between 0.15 and 0.35%,

manganese between 0.40 and 1.0%,

chromium between 0.40 and 1.10%,

nickel less than 4.5%,

molybdenum between 0.15 and 0.35%,

sulfur less than 0.040%,

phosphorus less than 0.035%, and

balance iron and other elements and compounds in making steel;

(b) preheating the provided steel composition to not more than 594° C.(1100° F.);

(c) heating the preheated steel composition to a peak temperature ofbetween 800° C. (1472° F.) and 1150° C. (2102° F.) in less than fortyseconds;

(d) holding the heated steel composition at the peak temperature rangefor between two and twenty seconds;

(e) quenching the heated steel composition from the peak temperaturerange to below 177° C. (350° F.) at a temperature rate reduction ofbetween 200 and 3000° C./sec (360-5400° F./sec); and

(f) air cooling the steel composition to less than 100° C. (212° F.) toform a steel article having at least 80% martensite and up to 5% bainiteby weight, a yield strength of at least 160 Ksi (1100 MPa), and a totalelongation between 5% and 22%.

Additionally, the air cooled steel composition may have a V₅₀ protectionballistic limit at 30° obliquity angle at least 2300 feet per second(701 m/s) with a 0.30 caliber armor piercing round for a thickness of0.25 inches (6.35 mm).

Also, disclosed is a method for treating a steel article to form a highyield strength and ductile alloy comprising the steps of:

(a) providing a steel composition having a material thickness no greaterthan 0.5 inches (12.7 mm), having an initial microstructure of at leastferrite and pearlite, and having a composition of, by weight,

carbon between 0.25 and 0.55%,

silicon between 0.15 and 0.35%,

manganese between 0.40 and 1.0%,

chromium between 0.40 and 1.10%,

nickel less than 4.5%,

molybdenum between 0.15 and 0.35%,

sulfur less than 0.040%,

phosphorus less than 0.035%, and

balance iron and other elements and compounds in making steel;

(b) preheating the provided steel composition to not more than 594° C.(1100° F.);

(c) heating the preheated steel composition to a peak temperature ofbetween 800° C. (1472° F.) and 1150° C. (2102° F.) in less than fortyseconds;

(d) holding the heated steel composition at the peak temperature rangefor between two and twenty seconds;

(e) quenching the heated steel composition from the peak temperaturerange to below 177° C. (350° F.) at a temperature rate reduction ofbetween 200 and 3000° C./sec (360-5400° F./sec);

(f) removing residual quench media from the surface of the quenchedsteel composition; and

(g) air cooling the steel composition to less than 100° C. (212° F.) toform a steel article having at least 80% martensite and up to 5% bainiteby weight, a yield strength of at least 160 Ksi (1100 MPa), and a totalelongation between 5% and 22%.

Additionally, the air cooled steel composition may have a V₅₀ protectionballistic limit at 30° obliquity angle at least 2300 feet per second(701 m/s) with a 0.30 caliber armor piercing round for a thickness of0.25 inches (6.35 mm).

Also disclosed is a method for treating a steel article to form a highyield strength and ductile alloy comprising the steps of:

(a) providing a steel composition having a material thickness no greaterthan 0.5 inches (12.7 mm), having an initial microstructure of at leastferrite and pearlite, and having a composition of, by weight,

carbon between 0.25 and 0.55%

silicon between 0.15 and 0.35%,

manganese between 0.40 and 1.0%,

chromium between 0.40 and 1.10%,

nickel less than 4.5%,

molybdenum between 0.15 and 0.35%,

sulfur less than 0.040%,

phosphorus less than 0.035%, and

balance iron and other elements and compounds in making steel;

(b) preheating the provided steel composition to not more than 815° C.(1500° F.);

(c) heating the preheated steel composition to a peak temperature ofbetween 800-1150° C. (1472-2102° F.) in less than forty seconds;

(d) holding the heated steel composition at the peak temperature rangefor between two and twenty seconds;

(e) quenching the heated steel composition to below 177° C. (350° F.) inless than four seconds;

(f) removing residual quench media from the surface of the quenchedsteel composition;

(g) tempering the quenched steel composition at a temperature between100° C. and 704° C. (212-1300° F.) for less than ninety minutes; and

(h) air cooling the tempered steel composition to less than 100° C.(212° F.) having a transformed microstructure of at least 80% martensiteand up to 5% bainite by weight, a yield strength of at least 160 Ksi(1100 MPa), and a total elongation between 5% and 22%.

Additionally, the air cooled steel composition may have a microstructurewith no more than 1% bainite by weight.

Alternatively, disclosed is a method for treating a steel article toform a high yield strength and ductile alloy comprising the steps of:

(a) providing a steel composition having a material thickness no greaterthan 0.5 inches (12.7 mm), having an initial microstructure of at leastferrite and pearlite, and having a composition of, by weight,

carbon between 0.25 and 0.55%

silicon between 0.15 and 0.35%,

manganese between 0.40 and 1.0%,

chromium between 0.40 and 1.10%,

nickel less than 4.5%,

molybdenum between 0.15 and 0.35%,

sulfur less than 0.040%,

phosphorus less than 0.035%,

balance iron and other elements and compounds in making steel;

(b) preheating the provided steel composition to a temperature to notmore than 815° C. (1500° F.);

(c) heating the preheated steel composition to a peak temperaturebetween 800° C. (1472° F.) and 1150° C. (2102° F.) in less than fortyseconds;

(d) holding the heated steel composition at the peak temperature rangefor between two and sixty seconds;

(e) quenching the heated steel composition from the peak temperaturerange to below 100° C. (212° F.) at a temperature rate reduction ofbetween 200 and 3000° C./sec (360-5400° F./sec);

(f) removing residual quench media from the surface of the quenchedsteel composition;

(g) tempering the quenched steel composition at a temperature from 100°C. to 704° C. (212-1300° F.) for less than ninety minutes; and

(h) air cooling the tempered steel composition to less than 100° C.(212° F.) to form a steel article having at least 80% martensite and upto 5% bainite by weight, a yield strength of at least 160 Ksi (1100MPa), and a total elongation between 5% and 22%.

Alternatively, the air cooled steel composition has a V₅₀ protectionballistic limit at 30° obliquity angle of at least 2300 feet per second(701 m/s) with a 0.30 caliber armor piercing round for a thickness of0.25 inches (6.35 mm)

Additionally, the air cooled steel composition may have a microstructurewith no more than 1% bainite by weight.

Alternatively, disclosed is a method for treating a steel article toform a high yield strength and ductile alloy comprising the steps of:

(a) providing a steel composition having a material thickness no greaterthan 0.5 inches (12.7 mm), having an initial microstructure of at leastferrite and pearlite, and having a composition of, by weight,

carbon between 0.25 and 0.55%

silicon between 0.15 and 0.35%,

manganese between 0.40 and 1.0%,

chromium between 0.40 and 1.10%,

nickel less than 4.5%,

molybdenum between 0.15 and 0.35%,

sulfur less than 0.040%,

phosphorus less than 0.035%,

balance iron and other elements and compounds in making steel;

(b) preheating the provided steel composition to not more than 594° C.(1100° F.);

(c) heating the preheated steel composition to a peak temperature ofbetween 800-1150° C. (1472-2102° F.) in less than forty seconds;

(d) holding the heated steel composition at the peak temperature rangefor between two and sixty seconds;

(e) quenching the heated steel composition from the peak temperaturerange to below 177° C. (350° F.) in less than four seconds;

(f) removing residual quench media from the surface of the quenchedsteel composition;

(g) tempering the quenched steel composition at a temperature from 100°C. and 704° C. (212-1300° F.) for less than ninety minutes; and

(h) air cooling the tempered steel composition to less than 100° C.(212° F.) having a transformed microstructure of at least 80% martensiteand up to 5% bainite by weight, a yield strength of at least 160 Ksi(1100 MPa), and a total elongation between 5% and 22%.

The air cooled steel composition may have a microstructure with no morethan 1% bainite by weight.

Additionally, the air cooled steel composition may have a V₅₀ protectionballistic limit at 30° obliquity angle at least 2300 feet per second(701 m/s) with a 0.30 caliber armor piercing round for a thickness of0.25 inches (6.35 mm).

The steel composition may be heated in step (c) in less than twentyseconds. Further, the heated steel composition may be held at the peaktemperature range for between two and twenty seconds. Further, theheated steel composition may be quenched from the peak temperature rangeto below 177° C. (350° F.) at a temperature rate reduction of between200 and 3000° C./sec (360-5400° F./sec). Further, the residual quenchmedia may be removed from the surface of the quenched steel compositionby at least one of mechanical wiping, blown air, and combinationsthereof. Alternatively, the tempering step is performed using aconventional oven. The tempering step may be performed using acombination of conventional oven and induction heater. Additionally, thetempering step may be performed at between 100° C. (212° F.) and 704° C.(1300° F.).

Alternatively disclosed is a method for treating a steel article to forma high yield strength and ductile alloy comprising the steps of:

(a) providing a steel composition having a material thickness no greaterthan 0.5 inch (12.7 mm), having an initial microstructure of at leastferrite and pearlite, and having a composition of, by weight,

carbon between 0.25 and 0.55%

silicon between 0.15 and 0.35%,

manganese between 0.40 and 1.0%,

chromium between 0.40 and 1.10%,

nickel less than 4.5%,

molybdenum between 0.15 and 0.35%,

sulfur less than 0.040%,

phosphorus less than 0.035%,

balance iron and other elements and compounds in making steel;

(b) preheating the provided steel composition to not more than 594° C.(1100° F.);

(c) heating the preheated steel composition to a peak temperaturebetween 800-1150° C. (1472-2102° F.) in less than forty seconds;

(d) holding the heated steel composition at the peak temperature rangefor between two and sixty seconds;

(e) quenching the heated steel composition to below 177° C. (350° F.) inless than four seconds;

(f) removing residual quench media from the surface of the quenchedsteel composition; and

(h) air cooling the steel composition to less than 100° C. (212° F.)having a transformed microstructure of at least 80% martensite and up to5% bainite by weight, a yield strength of at least 160 Ksi (1100 MPa),and a total elongation between 5% and 22%.

In any of the embodiments, prior to heating the steel composition, twoor more lengths of steel plates may be welded together along the widthwith one or more welds to form a continuous series of steel plates.Further, the step of welding may include applying a weave weld bridgingbetween lengths of steel plate across the width of the steel plates.Further, the step of welding may include applying a weave weld bridgingbetween lengths of steel plate in three sections where the centerportion of steel plate is done first and the side portions are welded toprovide a weave weld across the width of the steel plates. In any event,a seam weld is applied over the weave weld across the width of the steelplates. Further, an indicia may be applied to the steel plate in advanceof the welding step to enable a vision system to identify the locationof end portions of lengths of the steel plates for the welding step.

The heating step may be performed using an induction heater. Thequenching step may be performed by flowing a quench medium over thesteel article at a rate of up to 900 gallons/min (3400 L/min). Thequench medium may be water. The quenching step may be performed in morethan 1 second and not more than 20 seconds. After the quenching step,the steel plate is cut into lengths at least at the seams while thesteel plate continuously moves along the conveyor.

The tempering step may also be performed using an induction heater. Thetempering step may be performed at between 100° C. (212° F.) and 704° C.(1300° F.) in a time between 1 and 20 seconds.

Additionally, the steel composition may have, by weight, carbon between0.25 and 0.40%. Alternatively, the carbon composition may be between0.40 and 0.55%.

Additionally, the air cooled steel composition may have a microstructurehaving no more than 1% bainite by weight.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatical plan view of the heat treatment system of thepresent disclosure,

FIG. 2 is a diagrammatical side view of the heat treatment system ofFIG. 1,

FIG. 3 is a plan view of the pattern marked on a steel article to betreated for detection by a vision system;

FIG. 4 is a plan view of a welding pattern used for joining steelarticles to be treated by the disclosed method;

FIG. 5 is a photomicrograph showing the microstructure of a steelarticle prior to treatment according to the disclosed method;

FIG. 6 is a chart showing the effect of post-quench temperingtemperature on tensile strength on a steel article treated according tothe disclosed method;

FIG. 7 is a chart showing the effect of post-quench temperingtemperature on percent elongation on a steel article treated accordingto the disclosed method;

FIG. 8 is a photograph showing a cross-section of a steel articletreated according to the disclosed method following fracture in atensile test;

FIG. 9 is a photograph showing a cross-section of another steel articletreated according to the disclosed method following fracture in atensile test;

FIG. 10 is a chart showing the effect of post-quench temperingtemperature on ductility on a steel article treated according to thedisclosed method;

FIG. 11 is a photomicrograph showing the microstructure of a steelarticle treated according to the disclosed method;

FIG. 12 is a photomicrograph showing the microstructure of a steelarticle treated according to the disclosed method;

FIG. 13 is a photomicrograph showing the microstructure of a steelarticle treated according to the disclosed method; and

FIG. 14 is a photomicrograph showing the microstructure of a steelarticle treated according to the disclosed method.

DETAILED DESCRIPTION OF THE DRAWINGS

The present method is directed to an induction heated, quenched, andinduction tempered steel article and a method of making such a steelarticle. The starting material for the steel article has a compositioncomprising carbon in a range from about 0.25% by weight to about 0.55%by weight, silicon in a range from about 0.15% by weight to about 0.35%by weight, manganese in a range from about 0.40% by weight to about 1.0%by weight, chromium from about 0.40% by weight to about 1.10% by weight,nickel less than 4.5% by weight, molybdenum between 0.15 and 0.35% byweight, sulfur less than 0.040% by weight, phosphorus less than 0.035%by weight, with the balance of the composition comprising iron and otherelements and compounds in making steel. This balance may includeimpurities and other ingredients, for example from steel scrap or wire,in making steel. Additionally, the steel article may have carbon in arange from about 0.25% by weight to about 0.44% by weight and manganesein a range from about 0.40% by weight to about 0.60% by weight, theother components having the same composition ranges. Steel materialhaving this composition may be referred to as AISI steel grade 4130.Alternatively, the steel article may have carbon in a range from about0.40% by weight to about 0.55% by weight and manganese in a range fromabout 0.75% by weight to about 1.00% by weight, the other componentshaving the same composition ranges. Steel material having thiscomposition may be referred to as AISI steel grade 4140. Stated in termsof commercial grades, AISI steel grades from the 10XX family such as1020, 1030, 1040 and 1050, the 41XX family such as 4130 and 4150, the43XX family such as 4340, and the 86XX family such as 8630 and 8640 maybe used. Further, as described above, higher carbon steel grades, suchas ultra hard steel having up to 0.55% carbon, may be used with thedescribed invention.

Referring now to FIGS. 1 and 2, a heat treatment system 100 isillustrated that comprises a main machine frame 110 supported from afactory floor and supporting a discontinuous conveyor 200. The conveyor200 includes an entrance conveyor 210, where a starting material for asteel article to be treated by the system 100 is loaded, and an exitconveyor 240, where treated steel articles are removed from the systemand stacked at stacker 250. The entrance conveyor 210 and the exitconveyor 240 are aligned and spaced apart so as to accommodate theprovision of a heat treatment unit 300 in line between the two conveyors210, 240. The starting material for the steel article is initiallyprovided in the as-cast or as-rolled condition and may be subjected tospheroidized or non-spherodized annealing heat treatment. Whennon-spherodized, the initial material microstructure of the startingmaterial for steel article may have a non-annealed microstructure andmay have at least 90-97% ferrite and 2-10% pearlite, by weight, as shownin FIG. 5. Depending on the method of manufacturing of the steelarticle, the initial microstructure may have a banded structureconsistent with rolling.

Accordingly, a starting material for a steel article to be treated maybe loaded at the entrance conveyor 210, processed by the heat treatmentunit 300, transited down the exit conveyor 240, and stacked by thestacker 250, in a continuous process. This linear alignment of theconveyors 210, 240 and the heat treatment unit 300 facilitates rapidheat treatment of steel sheet, slab, and plate.

In operation, a starting material for the steel article to be treated,which may be provided, for example in the form of a sheet or plate, isloaded onto the entrance conveyor 210. The method is described in termsof processing a steel plate to form the steel article, but the form ofthe starting material in other forms, including without limitation steelslabs and steel sheet, as well as coiled product. In one instance, astarting material for the steel plate has a thickness of 0.50 inches(12.7 mm) or less, a length of 20 feet (6.1 m), and a width of 4 feet(1.2 m). The steel plate then begins to transit horizontally along thelength of the entrance conveyor 210 toward the heat treatment unit 300.Once the steel plate has moved a distance approximate to its length, forexample 20 feet (6.1 m), down the entrance conveyor 210, another steelplate is loaded on the entrance conveyor with the leading edge portionsof that steel plate abutting the trailing edge portions of the firstloaded steel plate. This process of advancement and loading of abuttingsteel plates may be carried out continuously so as to provide anuninterrupted run of steel plates to the heat treatment unit 300. So asto minimize inconsistent alignment from plate to plate through thesystem 100, an automated welder 220 may be provided on the entranceconveyor 210 and utilized to weld consecutive, abutting steel platestogether along their width to form a continuous series of steel plates.The welds may be evenly spaced along the width of the steel plates, andin one instance, the welder may make five welds along the width of thesteel plates. Alternatively, instead of the form of individual plateshaving fixed width and length, the starting material to be treated maybe provided in the form of a continuous sheet (e.g. a coil) located inline with the entrance conveyor 210 and fed continuously onto theentrance conveyor for subsequent treatment by the heat treatment unit300.

The steel plates may be in continuous motion at a substantially constantspeed along the conveyor 210 to facilitate the heating and quenchingprocesses. Welding the steel plates together as they contact on theconveyor 210 prevents the steel plate from shifting position oroverlapping each other as they move down the conveyor. This allows avision system 225 and welding robot 220 to provide a consistent weldjoint between lengths of steel plates. It also limits imperfections inthe steel plate going through drive pinch rolls 302 and 304, whichassists to maintain line speed as the welded seams move through thepinch rolls. Initial welding also allows the system to bridge gaps atthe seam between lengths of steel plates, further improving the weldingrobot weld process.

The vision system 225 for the welding robot may identify an indiciapattern of lines applied to or placed adjacent to the trailing edge ofeach steel plate, which may, for example, include a line 227 drawnacross the full width of the steel plate with two spaced apart smallerlines 229 substantially parallel to that line, and a dark area betweenspaced apart lines. This is an example of a pattern of indicia thatenables for correct position of lengths of steel plates to be recognizedby the vision system 225. Once the vision system 225 detects theindicia, it begins counting to signal the welding robot 220 to start theprogrammed weld process once the steel plate lengths are within the workarea. FIG. 3 illustrates the indicia recognized by the welding robotvision system 225. The vision system 235 for the plasma cutting robot230 may recognize a position for a welded seam across the full width ofthe steel plate. The scores and emphasis area of indicia is specific toavoid stray lines on the steel plate to be picked up and mistaken for aweld area. If that were to occur, the plasma cutting robot 230 may cutat that stray lines and disrupt the steel lengths through the systemuntil the next seam is detected.

The welding robot 220 may have a multiple pass program that is triggeredby the vision system 225 and encoder wheel that counts distancetravelled in millimeters tracking along the conveyor 200, to engage aweld program once the seam is within the robot work area. The robot workarea is based on points that are taught or touched on within the weldingprogram. The weld program may utilize three (3) separate welding weavepatterns, starting with the center portion of the steel plate 410,moving to first side portion of the steel plate 420, and then to asecond side portion of the steel plate as shown FIG. 4. The weavepatterns produced may be as shown in FIG. 4 before switching over to acover pass (or seam weld) welding the seam across the full width of thesteel plate as also shown in FIG. 4. This multiple pass pattern improvesthe weld process by first using a weave to bridge any gaps where steelplates meet before the final cover pass. The weave passes also heat thesteel plate before the final cover pass, which uses more wire and heatto penetrate the steel plate, thereby strengthening the weld seam sothat the continuous steel plate can move through the process withoutbreaking or otherwise becoming misaligned.

In order for the material to move at a more constant speed throughoutthe heating process there is a timed pressure relief at both the entry302 and exit 304 pinch rolls, which will allow the welded seam to passthrough the pinch rolls without slowing the material speed. This isachieved by the welding robot sending a pulse signal to the pinch rollcontrol system once it has completed its weld sequence. This pulsesignal will tell the pinch roll control system to start counting thedistance the material travels utilizing the encoder wheel on the pinchroll motor. As the welded seam travels down the line, the pinch rollswill relieve pressure once the welded seam reaches each pinch roll. Thisreduced pressure will allow the welded seam to roll through the pinchroll without stalling or slowing the line speed. The pinch rolls do notopen or lift off the material, but does have a secondary pressuresetting which reduces the pressure applied by the pinch rolls andtherefore allows the weld to roll through the pinch rolls. This systemhas been installed with each setting having adjustability to achievedesired performance. The following settings can be adjusted: primarypinch roll pressure; secondary pinch roll pressure; distance from thewelding robot to the entry pinch rolls; distance from entry to exitpinch roll; and the distance the material travels or window while thepressure is relieved.

The heat treatment unit 300 may include one or more preheat inductioncoils 301, a set of entrance pinch rolls 302, which guide the steelplate to be treated through one or more induction heat coils 310, aquench head 320, and a quench media removal unit 330, until theintermediate treated steel article to be formed from the starting steelplate is received by a set of exit pinch rolls 304. Similarly, the exitpinch rolls 304 serve to guide the steel plate through the inductiontempering coil 340 and onto the exit conveyor 240. Optionally, both theentrance pinch rolls 302 and the exit pinch rolls 304 may contain spacedcircumferential grooves, preferably equally spaced, corresponding to thespaced welds along the width of the steel plates. Such circumferentialgrooves provide relief into which any material built up during thewelding operation may be recessed as the welded portion of the steelplates pass between the pinch rolls 302 and 304.

Before entering the entrance pinch rolls 302, the steel plate may bepreheated in a preheat induction coil 301 while moving along theconveyor 200. The pre-heat power supply may be set, for example, to turnon 75 seconds after starting movement of the steel plate through theconveyor 200. At a conveyor speed of up to 75 inches per minute (1.9m/min), this involves moving along the conveyor up to 7.8 ft (2.4 m).Once on, the pre-heat power supply 360 may start at 1% and ramp up0.5-10% per second until it reaches a final power setting of below 120%of the power of the preheat induction coil. This ramp up involves thesteel plate moving another four to five feet of travel along theconveyor 200 before the pre-heat power supply reaches a operating powerlevel where the steel plate may reach a temperature of not more than815° C. (1500° F.) across the width of the steel plate. Alternatively,the steel plate may reach a temperature of up to 704° C. (1300° F.). Theramp up procedure for the power supply allows substantially evenly andgradually heating the steel plate through induction heating and aids incontrolling the shape and flatness of the steel plate with gradualheating to above 800° C. (1472° F.) before entry the rapid heatingsequence upon entry pinch rolls 302. Alternatively, the plate ispreheated to not more than 594° C. (1100° F.). The steel may bepreheated at a rate of 9-40° F./sec (522° C./sec).

The steel plates to be treated pass through the entrance pinch rolls 302and through one or more induction heating coils or an inductor 310 whichis powered by a power supply 315. The one or more induction heatingcoils 310 may be encased in concrete or other non-conductive material inorder to reduce damage to the induction coils as much as possible andreduce misaligned steel plates from passing through the coils, althoughnon-encased induction heating coils may also be provided. As the steelplate to be treated passes through the induction heating coils 310, aneddy current is induced in the steel plate, and it is the resistance ofthe steel material in conjunction with the eddy currents which heat thematerial. Given the configuration of the induction coils 310, the shapeof the steel plate passing through the coil, and the speed at which thesteel plate is moving through the heating coil, the steel material isheated to a temperature of between 800° C. and 1150° C. (1472-2102° F.)in ten seconds or less. Alternatively, the steel plate may be heated bythe heating coil to the same peak temperature range in forty seconds orless, or still further, in twenty seconds or less, as desired.

The induction heating may be accomplished by providing a number ofinduction heating coils in a series so that the steel plate passes undereach coil sequentially. According to one embodiment, each individualinduction heating coil may have a width of approximately 4 inches (102mm). Alternatively, the induction heating coil is an austenizing coilhaving a width of 8″. In one series arrangement, between 1 and 5 heatingcoils each having a width of 4″ (102 mm) are provided, providing aheating coil assembly having a width between 4 inches and 20 inches(102-508 mm). At a travel rate of approximately 40-75 inches per minute(1.0-1.8 m/min), the steel may take between 3.2 and 30 seconds tocompletely pass through the induction heating coils.

The induction heaters may have a ramp-up heating profile for heating thesteel plate. This may be accomplished by providing each inductionheating coil at a set temperature, providing a heating cycle for eachinduction heating coil, or other means of rapidly increasing thetemperature of the steel plate to a peak temperature range of between800° C. (1472° F.) and 1150° C. (2102° F.).

Following rapid induction heating, the heated steel plate is held at thepeak temperature range for between two and twenty seconds as it travelsto the quenching operation. Alternatively, the heated steel plate maytravel for between two and ten seconds. During this time, no additionalheat or other energy may be imparted to the steel plate, other than tomaintain temperature; nor is the steel plate subjected to any coolingmethod, other than exposure to the ambient atmosphere other than tomaintain temperature. For purposes of this disclosure, such time periodis referred to as holding the heated steel composition at the peaktemperature range, although it is expected that the steel plate willcool slightly during this period as it is no longer being heated by theinduction heater 310. According to a further embodiment, the heatedsteel composition may be held at the peak temperature range for betweentwo and sixty seconds. Alternatively, the heated steel composition maybe held at the peak temperature range for between two and thirtyseconds.

The heated steel plate then is subjected to a quenching operation as itpasses through a quench head 320 where a quench medium is flowed overthe steel plate at a rate of up to 900 gallons per minute (3400 L/min).The quenching operation decreases the temperature of the steel platefrom the peak temperature range of between 800° C. and 1150° C.(1472-2102° F.) to a temperature below 177° C. (350° F.), and may befrom 38° C. (100° F.) to 427° C. (800° F.) at a temperature reductionrate of between 200° C. per second and 3000° C. per second (360-5400°F./sec). The heated steel composition may be reduced to below 177° C.(350° F.) in less than four seconds. The quench medium, which in oneinstance may be water, is recycled through a quench media storage tank325 located adjacent the heat treatment unit 300. In addition to water,other quench media capable of achieving temperature reduction rates of200-3000° C./sec (360-5400° F./sec) may also be employed.

While little quench media will remain on the steel plate followingquenching, it is desirable to reduce, if not eliminate, any residualquench media on the steel plate prior to induction tempering bytechniques such as mechanical wiping, or forced air blowing, eitheralone or in combination. Accordingly, a quench medium reduction unit 330is provided in the heat treatment unit 300 following the quench head320. The quench medium reduction unit 330 may include wipers 332, airknives 334, and other drying apparatuses, either alone or incombination, so as to reduce the residual quench media on the steelplate prior to induction tempering. As the leading edge portions of thequenched steel plate passes through the quench medium reduction unit,the steel plate enters the exit pinch rolls 304, which serve to guidethe steel plate through the induction tempering coil 340 and onto theexit conveyor 240. Optionally, both the entrance pinch rolls 302 and theexit pinch rolls 304 may contain spaced circumferential grooves,preferably equally spaced corresponding to the spaced welds along thewidth of the steel plates. Such circumferential grooves may providerelief into which any material built up during the welding operation maybe recessed as the welded portion of the steel plate passes between thepinch rolls 302 or 304. The quenching step is performed in more than 1second and not more than 20 seconds.

After the residual quench media has been removed from the steel plate,the steel plate is then passed through an induction tempering coil 340to reduce any internal stresses that may have been introduced duringquenching. As with the induction heating coil 310, the inductiontempering coil 340 may optionally be encased in concrete or othernon-conductive material in order to minimize damage to the coil aspossible misaligned steel plates passes through the coil.

During the tempering step, the steel plate to form the steel article isheated to a temperature between 100° C. and 704° C. (212-1300° F.) andtempered for a period less than ninety minutes. Alternatively, the steelarticle may be heated to a temperature between 100° C. and 427° C.(212-800° F.) for less than ninety minutes. Three methods of temperingare contemplated. In a conventional oven tempering process, the steelarticle is heated to the desired temperature for less than 90 minutesand preferably less than 30 minutes. In an induction tempering process,the steel article is heated to the temperature range for less than 10minutes and preferably less than 2 minutes. The tempering process may beas short as between 1 and 20 seconds at between 100° C. and 704° C.(212-1300° F.). In a combination induction and conventional oventempering process, the steel article may be heated to the desiredtemperature for less than 60 minutes and preferably more than 30minutes. As with the induction heating coil 310, the induction temperingcoil 340 is powered by its own distinct power supply, i.e. inductiontempering coil power supply 345, located proximate the heat treatmentsystem 100. Following tempering, the tempered steel plate is thendischarged onto the exit conveyor 240, which is provided with a cuttingdevice 230.

The cutting device 230 may be a plasma torch, an oxy-fuel torch, orother cutting apparatus which may be affixed to an articulated roboticarm configured to cut the moving steel plate into desired lengths as theplate travels down the exit conveyor 240. A plasma cutting robot 230 mayhave two cutting and vision programs within its main program. At thestart of each run, the vision system 235 seeks the front edge of thesteel plate. Once the front edge is detected, the cutting robot 230utilizes an encoder wheel that counts the steel plate movement along theconveyor 240 and makes lead rip cuts at steel plate lengths. After theprogram has made one lead cut, the vision system programs identifies awelded seam and the plasma robot 230 cuts the steel plate on that seamacross its width, and then waits, tracking the steel plate movementalong the conveyor with the encoder wheel to make another lead rip cutbefore switching back to the vision system to identify the next weldedseam and making the next cut. This process continues for the duration ofa run as steel plate is cut into substantially rectangular lengths atleast at the seams while the steel plate continuously moves along theconveyor. In one example, the steel plate may be cut into four feet (1.2m) wide by ten feet (3.0 m) long segments, although other lengths andwidths may also be desirable depending upon the ultimate application forthe steel article.

Following cutting, the tempered steel plate is air cooled as it passesdown the exit conveyor to reach a temperature of less than 100° C. (212°F.). The steel articles may then be stacked into a stack by a stacker250 and subsequently transported to another location.

Following heat treatment and quenching as described above, themechanical properties of the steel article may be tailored to desiredspecifications by changing the tempering temperature of the processbetween 100° C. and 704° C. (212° F.-1300° F.). According to oneembodiment the tempered steel composition may have at least 80%martensite and up to 5% bainite by weight and a yield strength of atleast 160 ksi (1100 MPa) and a total elongation between 5% and 22%.

As shown in FIG. 6, we have found an indirect relationship betweentensile strength and tempering temperature. For example, tempering at260° C. (500° F.) resulted in tensile strength of 260.5 ksi (1796 MPa),while tempering at 200° C. (392° F.) resulted in a tensile strength of275.3 ksi (1898 MPa), a difference of 14.8 ksi (102 MPa) orapproximately 5%. Turning to FIG. 9, the relationship between percentelongation and tempering temperature is indicated. Notably, increasingthe tempering temperature from 200° C. to 220° C. (392-428° F.)decreased the percent elongation of the treated steel samples, butfurther increases in the tempering temperature increased the percentageof elongation at 260° C. (500° F.) tempering temperature was the same asthe percentage of elongation observed in the sample tempered at 204° C.(400° F.).

The ductility was evaluated again after induction tempering attemperatures between 204° C. and 260° C. (400-500° F.) using a testmethod based on the ASTM E-8 method for ductility determination. In thismethod the ductility measurement, designated as the percentage of areareduction, is represented as the ratio of the cross-sectional area ofthe sample at the tensile break to the original cross-sectional areatimes 100. Thus, a lower percentage represents an increased amount ofductility. The 204° C. (400° F.) and 260° C. (500° F.) temper annealedsamples, represented in FIGS. 6 and 7, show that in contrast to thepercent elongation measured during the tensile testing there was adirect relationship between the temper temperature and the percentage inarea reduction, i.e., tempering at 260° C. (500° F.) temperaturesresulted a lower percentage in area reduction (58.6%) than tempering at204° C. (400° F.) (69.7% area reduction).

The steel article formed and treated by the disclosed method may beemployed in armor applications. In particular, the steel article formedand treated by the disclosed method has also been subjected toballistics testing according to standards set forth in MIL-DTL-32332,MIL-DTL-46100E, MIL-DTL-12560J (classes 1 and 4), as well as NIJ threatlevel 3. The results from such ballistics testing of the steel articleare tabulated in Table 1. The results show comparison of a 0.25 inch(6.35 mm) thick sample of AISI 4130 steel treated by the presentlydisclosed method compared to other standard materials used in armorapplications.

TABLE 1 (V50 protection ballistic limit (fps)) Thickness .030-cal M2AP20 mm FSP Material (inches) @ 30 deg. @ 0 deg. 4130 Steel Treated By0.250 2461 1800 Disclosed Process RHA 360 Hb 0.250 2100 1544 HighHard500Hb 0.250 2300-2400 <1500 est. 5083 Al 0.733 — 1200 5059 Al 0.733 18401200 MgAz31B—H24 1.125 No Data 1300 Ti—6Al—4V 0.444 No Data 1550

For purposes of this disclosure, the V₅₀ protection ballistic limit isdefined as the average of six fair impact velocities comprising thethree lowest velocities resulting in complete penetration and the threehighest velocities resulting in partial penetration of the testspecimen, as further explained in MIL-DTL-32332 and MIL-DTL-46100E (MR)with Amendment 1 of 24 Oct. 2008, which is incorporated by reference inits entirety herein. Table 1 shows that steel plate formed and treatedby the presently disclosed method exhibits V₅₀ values which are the sameas or exceed V₅₀ values for the comparative materials of similarthickness. As such, it may be possible to use a relatively thinnercladding of steel plate formed and treated by the current method toachieve at least the same level of ballistic protection. Therefore, theweight of a vehicle clad with steel articles treated by the disclosedmethod may be relatively lighter as compared to vehicles clad with thecomparative armor materials. Thus, steel articles formed and treated bythe present method may result in relatively improved fuel economy,transportability, maneuverability, and other benefits of a generallylighter weight vehicle. Such data is summarized below in Table 2, whichshows the thickness in inches required for each of the tested steelplate to achieve passing ballistics results for a 2100 feet per second(640 m/s) projectile and the corresponding pounds per square foot foreach armor material for the required thicknesses. According to variousembodiments of the invention, the V₅₀ protection ballistic limit at 30°obliquity angle is at least 2300 feet per second (701 m/s) with a 0.30caliber armor piercing round for a thickness of 0.25 inches (6.35 mm).

TABLE 2 10.2 psf 10.2 psf Material V50 in fps Thickness 4130 SteelTreated By 2255 0.250″ Disclosed Process RHA 360 Hb 1700 0.250″ HighHard500Hb 1640 0.250″ 5083 Al 1625 0.733″ 5059 Al 1675 (est) .0733″MgAz31B—H24 1650 1.125″ Ti—6Al—4V 1910 0.444″

The microstructure of steel article has a direct relationship on themechanical properties. Accordingly, the microstructure of the steelspecimens formed and treated by the disclosed method were also examined,both before and after heat treatment. FIG. 5 shows the initialmicrostructure of the starting material for the steel article prior totreatment is ferrite and pearlite. Following formation and treatment bythe disclosed method, the microstructure of the steel article may be 80percent or more martensite and up to 5 percent bainite by weight, andmay approach 100 percent martensite by weight. The microstructure of thesteel article includes less than 5% bainite (by weight) and may includeless than 1% bainite (by weight), or as little as trace amounts. FIGS.11 and 12 show the microstructure of steel article formed and temperedat 260° C. (500° F.) as described above after nital etching at 500 and1000 times magnification, respectively. Similarly, FIGS. 13 and 14 showthe microstructure of steel article formed and tempered at 204° C. (400°F.) after nital etching at 500 and 1000 times magnification,respectively. In both instances, the analysis shows that themicrostructure of the samples of the steel article consisted of almostentirely of martensite.

The above-described process can also be used to produce high hard steeland ultra-high hard steel by the inclusion of nickel in the providedsteel composition. The amount of nickel may range from 0-4.5%, whereingreater proportions of nickel increases the ductility of the steelproduct. The high hard and ultra high hard steel products may includeyield strengths as shown below in Table 3, wherein various tensileproperties are shown.

TABLE 3 Average Yield Average Ultimate Yield Strength Ultimate StrengthStrength Range Strength Range High 190 ksi 160-205 ksi 240 ksi 210-255ksi Hard Ultra 230 ksi 210-250 ksi 325 ksi 300-345 ksi High Hard High1310 MPa 1103-1413 MPa 1655 MPa 1448-1758 MPa Hard Ultra 1586 MPa1448-1724 MPa 2241 MPa 2068-2379 MPa High Hard

While the invention has been described with reference to certainembodiments it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the invention without departing from its scope.Therefore, it is intended that the invention not be limited to theparticular embodiments falling within the scope of the appended claims.

What is claimed is:
 1. A method for treating a steel article to form ahigh yield strength and ductile alloy comprising the steps of: (a)providing a steel composition having a material thickness no greaterthan 0.5 inches (12.7 mm), having an initial microstructure of at leastferrite and pearlite, and having a composition of, by weight, carbonbetween 0.25 and 0.55% silicon between 0.15 and 0.35%, manganese between0.40 and 1.0%, chromium between 0.40 and 1.10%, nickel less than 4.5%,molybdenum between 0.15 and 0.35%, sulfur less than 0.040%, phosphorusless than 0.035%, balance iron and other elements and compounds inmaking steel; (b) preheating the provided steel composition to not morethan 594° C. (1100° F.); (c) heating the preheated steel composition toa peak temperature of between 800° C. (1472° F.) and 1150° C. (2102° F.)in less than forty seconds; (d) holding the heated steel composition atthe peak temperature range for between two and twenty seconds; (e)quenching the heated steel composition from the peak temperature rangeto below 177° C. (350° F.) at a temperature rate reduction of between200 and 3000° C./sec (360-5400° F./sec); (f) tempering the quenchedsteel composition at a temperature from 100° C. to 704° C. (212-1300°F.) for less than ninety minutes; and (g) air cooling the tempered steelcomposition to less than 100° C. (212° F.) to form a steel articlehaving at least 80% martensite and up to 5% bainite by weight, a yieldstrength of at least 160 Ksi (1100 MPa), and a total elongation between5% and 22%.
 2. The method for treating a steel article as claimed inclaim 1 where the air cooled steel composition has a V₅₀ protectionballistic limit at 30° obliquity angle at least 2300 feet per second(701 m/s) with a 0.30 caliber armor piercing round for a thickness of0.25 inches (6.35 mm).
 3. The method for treating a steel article asclaimed in claim 1, where the air cooled steel composition has amicrostructure having no more than 1% bainite by weight.
 4. A method fortreating a steel article to form a high yield strength and ductile alloycomprising the steps of: (a) providing a steel composition having amaterial thickness no greater than 0.5 inches (12.7 mm), having aninitial microstructure of at least ferrite and pearlite, and having acomposition of, by weight, carbon between 0.25 and 0.55% silicon between0.15 and 0.35%, manganese between 0.40 and 1.0%, chromium between 0.40and 1.10%, nickel less than 4.5%, molybdenum between 0.15 and 0.35%,sulfur less than 0.040%, phosphorus less than 0.035%, balance iron andother elements and compounds in making steel; (b) preheating theprovided steel composition to not more than 594° C. (1100° F.); (c)heating the preheated steel composition to a peak temperature of between800° C. (1472° F.) and 1150° C. (2102° F.) in less than forty seconds;(d) holding the heated steel composition at the peak temperature rangefor between two and twenty seconds; (e) quenching the heated steelcomposition from the peak temperature range to below 177° C. (350° F.)at a temperature rate reduction of between 200 and 3000° C./sec(360-5400° F./sec); and (f) air cooling the steel composition to lessthan 100° C. (212° F.) to form a steel article having at least 80%martensite and up to 5% bainite by weight, a yield strength of at least160 Ksi (1100 MPa), and a total elongation between 5% and 22%.
 5. Themethod for treating a steel article as claimed in claim 4 where the aircooled steel composition has a V₅₀ protection ballistic limit at 30°obliquity angle at least 2300 feet per second (701 m/s) with a 0.30caliber armor piercing round for a thickness of 0.25 inches (6.35 mm).